This invention relates to a method and apparatus for drilling microvia holes in electrical circuit packages, and particularly to the use of mid-infrared pulsed laser radiation sources therein.
For the purposes of this invention the following definitions of spectral wavelength regions apply: ultraviolet: 190-400 nm, visible: 400 nm-700 nm, near-infrared: 700 nm-1.5 cm, mid-infrared: 1.51 xcexcm-8 xcexcm, far-infrared: 8-100 xcexcm.
Since 1981, pulsed lasers have been investigated for the purposes of drilling small interconnection (microvia) holes in circuit interconnection packages such as printed circuit boards or printed wiring boards (PCB""s or PWB""s), multichip modules (MCM""s), ball grid arrays (BGA""s), tapeautomated bonds (TAB""s), etc. A schematic depicting plated and unplated, blind and through, microvia holes drilled in a 4-level board is shown in FIG. 1. Pulsed far-infrared CO2 lasers between 9-11 xcexcm wavelength are now used to drill blind via holes in the electrically insulating dielectric material of the multilayer sandwich comprising these packages. Similarly near-infrared solid-state Nd lasers at 1.06 xcexcm wavelength are being used with some success to drill via holes in the top conductive copper layer of the package as well as the dielectric layer. Likewise frequency-tripled and quadrupled radiation at 355 nm and 266 nm from these lasers are also being used for this application. Although since 1987 ultraviolet excimer lasers at wavelengths of 248 nm and 308 nm have been used for manufacturing via holes in high-value MCM""s and BGA""s interconnect packages, their relatively slow drilling speeds coupled to their high capital and running costs make these lasers unsuitable for drilling vias in lower cost devices such as PCB""s and PWB""s.
Each pulsed laser has distinct advantages and disadvantages for drilling microvias. Due to the low powers ( less than 4W) available, high operating costs ( greater than $2/hr-dominated by replacement harmonic conversation crystal costs) and low ablation rates ( less than 1 xcexcm/pulse), the use of harmonically-generated ultraviolet radiation from 1.061 xcexcm Nd lasers for microvia drilling tends to be a relatively slow and expensive process. On the other hand, because of the relatively high near-infrared transparency of most organic materials used as dielectric materials, pulsed laser sources in this spectral region produce poor hole quality when drilling the dielectric layer and when drilling the top copper layer, dielectric materials underneath are sometimes prone to damage. Far-infrared lasers cannot effectively drill metallic copper due to plasma screening effects experienced by longer wavelength radiation and holes drilled in the dielectric tend to be of a shape and quality which make subsequent plating processes difficult, unless extra steps, such as oxidising the metal to increase its coupling efficiency, are used.
According to a first aspect of the present invention, there is provided a method of drilling microvia holes in printed circuit boards or printed wiring boards or other electrical circuit interconnection packages using a pulsed laser radiation source emitting radiation having a wavelength or wavelengths in the mid-infrared region.
The method may comprise using a pulsed laser radiation source comprising a Holmium laser, producing radiation at 2.1 xcexcm wavelength. The method may comprise using a pulsed laser radiation source comprising an Erbium laser, producing radiation at 2.94 xcexcm wavelength.
The method may comprise using a pulsed laser radiation source comprising a gas Carbon Monoxide (CO) laser, producing radiation having wavelengths in the range 5-6 xcexcm.
The method may comprise using a pulsed laser radiation source comprising a laser and an Optical Parametric Oscillator (OPO) or an Optical Parametric Amplifier (OPA), each tuneable between 1.5-4.8 xcexcm.
These laser radiation sources are suitable for this invention and for implementing into manufacturing processes, having high reliability, infrequent servicing, low capital and operating costs.
Preferably, the method comprises using a pulsed laser radiation source capable of producing an average laser radiation power of greater than 2W, and preferably having operating costs of less than $1/ hr.
Preferably, the method comprise s drilling microvia holes at speeds exceeding 200 holes/sec. The speed of drilling will depend on the tool architecture used, the material type, thickness and hole density.
The use of mid-infrared lasers for this method solves many of the difficulties encountered when using pulsed laser sources at other wavelengths. Mid-infrared lasers provide laser radiation sources capable of drilling high quality microvia holes at high drilling speeds with relatively low operating costs and high reliability.
Issues associated with laser drilling microvias in a multilayer sandwich of materials such as encountered in PCB""s can be broken down into several subproblems. This invention relates to solving each of these by appropriate use of one or more suitably chosen pulsed laser radiation sources emitting radiation in the mid-infrared spectral region.